1. Field of the Invention
This invention relates to a heating apparatus for drawing polyolefin fibers and a process for drawing such fibers.
2. Description of the Related Art
High tenacity polyolefin fibers, such as gel-spun polyethylene fibers, are known in the art. Ultrahigh molecular weight polyolefins include polyethylene, polypropylene, poly(butene-1), poly(4-methyl-pentene-1), their copolymers, blends and adducts. They are prepared from ultrahigh molecular weight polyolefins, and in the case of polyethylene, ultrahigh molecular weight polyethylene (UHMWPE). The preparation and drawing of such fibers have been described in various patent publications, including U.S. Pat. Nos. 4,413,110; 4,430,383; 4,436,689; 4,536,536; 4,545,950; 4,551,296; 4,612,148; 4,617,233; 4,663,101; 5,032,338; 5,246,657; 5,286,435; 5,342,567; 5,578,374; 5,736,244; 5,741,451; 5,958,582; 5,972,498; 6,448,359; 6,969,553 and U.S. patent application publication 2005/0093200, the disclosures of which are expressly incorporated herein by reference to the extent not incompatible herewith. An oven for drawing fibers is also disclosed in U.S. patent application publication 2004/0040176.
UHMWPE yarns are useful in many applications, such as in impact absorption and ballistic resistant products. These include body armor, helmets, aircraft shields and composite sports equipment. They are also useful in fishing line, sails, ropes sutures and fabrics.
In a typical drawing configuration, the gel-spun fibers are prepared by spinning a solution of ultrahigh molecular weight polyethylene, cooling the solution filaments to a gel state and then removing the spinning solution. The spun fibers are then drawn to a highly oriented state. In the drawing operation, typically the spun fibers are first fed to a first stack of heated rolls, then through one or more ovens (typically four), then to a second stack of heated rolls, then to one or more additional ovens (typically two), and finally to a third stack of heated rolls before the fiber or yarn is wound up. The speed and temperature of the rolls are adjusted, as are the temperature and temperature profile in the ovens, to obtain the desired drawing ratio and product characteristics in the fiber or yarn. The fibers are subjected to a two stage draw operation in accordance with this configuration.
Although such a configuration has produced excellent quality fiber and yarn, the overall operation is expensive due to the multiple heating zones and sets of rolls, and the throughput is restricted. It would be desirable to provide an oven configuration for polyethylene fibers which was less expensive to operate and could provide drawn fibers or yarns at a higher rate.